Preparation of tetrachlorinated pyridines is well known and such compounds are known to be useful as pesticides or as intermediates for the preparation of various pesticides. For example, 2,3,4,6-tetrachloropyridine is useful as a starting material for preparation of certain pesticides (see U.S. Pat. No. 3,549,647). It is recognized in the art that although methods are available for preparing tetrachlorinated pyridines, a major problem is to selectively prepare a substantially pure specific compound. It would be desirable to have a method that results in the production of substantially pure 2,3,4,6-tetrachloropyridine to avoid waste and to facilitate isolation of the desired product.
Many known methods for preparing tetrachlorinated pyridines provide such complex product mixtures and low yields of a specific compound as to render such methods unsuitable as synthetic methods for preparation of a particular compound. However, certain known liquid phase methods are purported to have good selectivity and yields for particular tetrachlorinated pyridine compounds. For example, U.S. Pat. No. 3,186,994 teaches preparation of 2,3,5,6-tetrachloropyridine and 2,3,4,5-tetrachloropyridine; by chlorinating in the absence of a catalyst, an appropriate polychloro-(trichloromethyl)-pyridine reactant in the liquid state at a temperature of at least 160.degree. C., preferably under irradiation with ultraviolet light. U.S. Pat. No. 3,732,230 teaches, inter alia, liquid phase chlorination of an appropriate pyridine hydrochloride to yield a product composition containing 2,3,4,5-tetrachloropyridine and other polychloropyridines. U.S. Pat. No. 4,256,894 teaches, inter alia, preparation, in the presence of a catalyst, of 2,3,5,6-tetrachloropyridine and pentachloropyridine.
None of the above-noted references teach a method for preparing 2,3,4,6-tetrachloropyridine. Such a method is an object of the present invention. Prior art methods which specifically teach preparation of 2,3,4,6-tetrachloropyridine have either described the 2,3,4,6-tetrachloropyridine to be one component of a complex mixture (see U.S. Pat. No. 3,420,833) or have been industrially less desirable methods such as diazotization of 3-amino-2,4,6-trichloropyridine (see J. Den Hertog, et al., Rec. Trav. Chim. 69 673, 1950 (Chemical Abstracts 44 8919)).